Since energy issues and global environmental issues are becoming more serious, solar cells are receiving more attention as alternative energy for replacing fossil fuels. In the solar cells, carriers (electrons and holes) generated by light irradiation to a photoelectric conversion section composed of a semiconductor junction or the like are extracted to an external circuit to generate electricity. A collecting electrode is provided on the photoelectric conversion section of the solar cell for efficiently extracting carriers generated at the photoelectric conversion section to the external circuit.
For example, in a crystalline silicon-based solar cell using a single-crystalline silicon substrate or a polycrystalline silicon substrate, a collecting electrode made of a fine metal is provided on a light-receiving surface. Also, in a heterojunction solar cell having amorphous silicon layers and transparent electrode layers on a crystalline silicon substrate, collecting electrode(s) are provided on the transparent electrode layer(s).
The collecting electrode of the solar cell is generally formed by pattern-printing a silver paste by a screen printing method. This method is simple in terms of the process itself, but has such a problem that the material cost of silver is high, and that the resistivity of the collecting electrode increases because a silver paste material containing a resin is used. For decreasing the resistivity of the collecting electrode formed of a silver paste, it is necessary to thickly print the silver paste. However, since the line width of the electrode increases with the increase of the print thickness, thinning of the electrode is difficult, and the shading area of the collecting electrode increases.
As a solution to these problems, a method is known in which a collecting electrode is formed by a plating method which is excellent in terms of material and process costs. For example, Patent Documents 1 to 3 disclose a solar cell in which a metallic layer made of copper or the like is formed by a plating method on a transparent electrode that forms a photoelectric conversion section. In this method, first, a resist material layer (insulating layer) having an opening matching the shape of a collecting electrode is formed on the transparent electrode layer of the photoelectric conversion section, and a metallic layer is formed at the resist opening section on the transparent electrode layer by electroplating. Thereafter, the resist is removed to form a collecting electrode having a predetermined shape. Patent Document 3 discloses that the line width of a plating electrode is made equal to or less than that of an under-layer electrode by forming the plating electrode layer using a mask after a formation of the under-layer electrode.
Patent Document 4 discloses a method in which an insulating layer of SiO2 or the like is provided on a transparent electrode, a groove extending through the insulating layer is then provided to expose the surface or side surface of the transparent electrode layer, and a metal collecting electrode is formed so as to be in conduction with an exposed area of the transparent electrode. Specifically, a method is proposed in which a metal seed is formed on the exposed area of the transparent electrode layer by a light induced plating method or the like, and a metal electrode is formed by electroplating with the metal seed as an origination point. This method is more advantageous in terms of material costs and process costs because it is not necessary to use a resist unlike Patent Documents 1 and 2. By providing a low-resistance metal seed, the contact resistance between a transparent electrode layer and a collecting electrode can be lowered.
Patent Document 5 discloses a method in which an electroconductive seed having proper roughness and porosity is provided, an insulating layer is formed on the electroconductive seed, discontinuous openings are formed in the insulating layer on the electroconductive seed, and a collecting electrode is formed by plating with the openings as origination points.